1. Field of the Invention
The invention relates to a quasi-optical component for microwave radiation comprising a quasioptical element which radiates incident microwave radiation along a major axis and which has a characteristic transverse dimension which is smaller than 50 times one wavelength.
2. Discussion of Background
For using microwaves for heating fusion plasmas, very high powers (1-30 MW) are needed in the range from approximately 50 GHz. As has been shown by studies, these powers can be best controlled by means of so-called quasi-optical components. The term quasioptical denotes the principle that microwaves are no longer conducted by conducting walls but propagate approximately under free-space conditions.
Such quasi-optical components can be used in the heating of plasmas by means of microwaves at different positions, for example in the microwave source (quasi-optical or also cylindrical gyrotron) or in the transmission system (compare "Design of the CIT Gyrotron ECRH Transmission System", J. A. Casey et al., 13th Int. Conf. on Infrared and Millimeter Waves, Dec. 5-9, 1988, pp. 123-124). In connection with the cylindrical gyrotron, the so-called Vlasov convertor, above all, is of significance. Such a quasi-optical element is described, for example, in the publications "An X-Band Vlasov-Type Mode Convertor", B. G. Ruth et al., 13th Int. Conf. on Infrared and Millimeter Waves, Dec. 5-9, 1988, pp. 119-120, and "A quasi-optical convertor for efficient conversion of whispering gallery modes into narrow beam waves", A. Mobius et al., 13th Int. Conf. on Infrared and Millimeter Waves, Dec. 5-9, 1988, pp. 121-122. The example of the gyrotron will be used for explaining that, however, quasi-optical components also entail problems.
In the gyrotron, an electron beam gun generates an electron beam which passes via a drift system into a resonator where a part of the kinetic energy of the electrons is converted into the desired microwave radiation.
The quality of the electron beam plays a central role for the optimum excitation of the microwaves. So that the beam quality along the drift system is impaired as little as possible, it must be ensured that the electrons are always subject to an electrical potential in this system. In principle, this can be achieved by means of a cylindrical or possibly conical metal tube which has a diameter which is a few millimeters larger than the electron beam.
However, this tube can also resonate. This would result in a dramatic deterioration in the beam quality. This is why suitable means must be used for ensuring that no microwaves can be generated in this area. In addition, this area has the task of damping microwaves which pass from the resonator to the gun.
At present, there are two solutions for this problem. One is provided in the published Patent Application EP-0 301 929 Al. In this publication, in the case of a cylindrical gyrotron a conical beam guide with ribbed metallic inner surface is arranged in the drift system. Between the metal ribs protruding toward the inside, absorbing rings of magnesium oxide are arranged.
This solution has the following principle of operation. The copper ring protruding slightly toward the inside forms the electrical surface. The damping ring behind it does not influence the electron movement but damps the microwaves. The disadvantage of this solution which is used in most cases is the high price of the damping ceramics and the poor thermal coupling of the ceramics to a heat sink. In addition, the interior of this beam conductor cannot be easily pumped.
The second solution is known from the Patent CH-664,044 A5. The electrically conducting surface of the beam guide is here achieved by a metal grid enclosing the electron beam. The structure is provided with the characteristic of resonance damping by means of the penetrations in the grid. They are dimensioned in such a manner that they pass the microwaves to be damped. In this solution, the undefined absorption of the microwaves represents a problem.
Further problems occur in conjunction with microwaves which are coupled back from the resonator into the electron beam space and can develop a similarly disturbing effect.